This collection of eighteen essays in honor of Paul N. Rosenstein-Rodan includes contributions from an international group of distinguished economists - in economic development these are represented by Everett E. Hagen, Bent Hansen, I. M. D. Little, James Mirrlees, and Jan Tinbergen; in economic theory by Franco Modigliani, Paul A. Samuelson, and Robert M. Solow; and in economic policy by John H. Adler, Alexander Kafka, C. P. Kindleberger, and Pasquale Saraceno.The essays cover some of the central issues of national and regional development, international trade, and resource allocation. Two introductory essays review the evolution of concepts and policies in development. Several others present models that provide new formulations of economic growth processes and take up issues of development planning. These are followed by essays that penetrate below the aggregates of national development and examine regional growth and income distribution. Three essays discuss aspects of international capital movements and trade in relation to development, while others focus on the evaluation of investment projects and the analysis of labor productivity differences.